This invention relates to an audio power amplifier for stereophonic systems, which can be operated either in class A, class B or class AB amplifier operation mode, and in which the working power supply voltage can also be switched to levels convenient for the respective class A, class B and class AB operation modes.
The transistorized power amplifiers heretofore used for high fidelity amplifiers are mostly of the push-pull type, and their operation mode is usually either of class A amplifier, class B amplifier or class AB amplifier type.
Of the above amplifiers the class B power amplifier is highly efficient and permits obtaining a large output, so that this type of amplifier has been adopted for almost all transistorized power amplifiers. On the debit side, however, it is prone to crossover distortions, deteriorating the quality of sound produced.
On the other hand, the class A power amplifier is less prone to the crossover distortion so that it is superior to the class B power amplifier so far as the sound quality is concerned. However, in comparison to the class B power amplifier, the idling current is so increased that the efficiency is inferior, and the working region is so narrow that a large output cannot be obtained.
The class AB power amplifier has the demerits of both the afore-mentioned two types of amplifiers, so that it is hardly adopted, particularly for transistorized power amplifiers, but it is sometimes adopted in case of vacuum tube amplifiers.
Accordingly, it has been in practice to provide class A, class B or class AB amplifier circuits separately so that a single amplifier unit includes only one mode amplifier, therefore a user should select either one out of separate models according to the use of the amplifier. For example, when it is desired to produce sound of small volume and high quality by using an efficient loudspeaker, the class A power amplifier circuit is used; while for producing sound of large volume from a loudspeaker irrespective of low efficiency or high efficiency, the class B power amplifier circuit is used.
Therefore, in order for a single audio amplifier to be capable of operation in any one of the class A, class B and class AB modes it has heretofore been necessary to assemble amplifier circuits of all these classes in the same amplifier unit, which has been very uneconomical.
An object of this invention, accordingly, is to provide an economical power amplifier for stereophonic systems or the like, which is simple in construction and can be operated either as class A, class B or class AB amplifier by selecting a corresponding one of bias current levels set in a bias setting circuitry provided in a driver stage driving the main power amplifier stage.
Another object of the invention is to provide a power amplifier, which is provided with means for always maintaining the null potential on its output terminal in the vicinity of zero volt irrespective of the operation mode to prevent d-c current from flowing into the loudspeaker to thereby prevent the burning thereof.
A further object of the invention is to provide a power amplifier for stereophonic systems, which permits to select class A, class B and class AB operation modes of its power amplifier stage through mere switching operation of a selector switch.
A still further object of the invention is to provide a power amplifier for stereophonic systems, which is provided with means for automatically switching working source voltages corresponding to the class A, class B and class AB operation modes of the power amplifier stage and associated circuit in an interlocked relation to the switching action of the select switch.
A yet further object of the invention is to provide a power amplifier for stereophonic systems, in which the selector switch is adapted to be short-circuited for a certain period when it is switched for switching the class B amplifier operation mode of the power amplifier stage over to the class A or class AB operation mode to thereby protect the power amplifier stage elements.
The power amplifier for stereophonic systems featured by the invention comprises a power amplifier stage, a driver stage for driving the power amplifier stage, and a bias setting circuitry including switching means for selectively switching bias values to be supplied to the driver stage for rendering class A, class B and class AB amplifier operation modes of the amplifier stage to thereby select one of the operation modes.